Switches can be used as building blocks in many different circuits. When turned on, a switch can generally connect an input to an output to allow a signal at the input to pass through to the output. When turned off, the switch can disconnect the input from the output to prevent the signal at the input from passing through to the output. Individual switches can be implemented by a variety of circuits, with one of the simplest including only single transistor. Two main characteristics of a switch are its on-resistance and its leakage current.
The on-resistance of a switch can be the resistance the switch presents in series between the input and output when the switch is turned on. It is often desirable that the on-resistance be as low as possible, and ideally be zero, so that the switch effectively presents a short circuit when it is turned on, directly connecting the input to the output with no resistance in between. It is also often desirable that any on-resistance be independent of the signal being transmitted by the switch, so that the switch does not introduce non-linear or other undesirable effects onto the transmitted signal. However, in reality, there is often a finite on-resistance presented by the circuit implementing the switch, and this on-resistance can vary based on the signal being transmitted by the switch.
The leakage current of a switch can be the amount of current that leaks between the input and output terminals when the switch is turned off. It is often desirable that leakage current be as low as possible, and ideally be zero, so that the switch effectively presents an open circuit when turned off, completely cutting off the input from the output with an infinite resistance in between. Another way of characterizing the leakage current is as an off-resistance of the switch. To achieve a zero leakage current, a switch circuit should have an infinite off-resistance. However, in reality, there is often a finite leakage current exhibited by the circuit implementing the switch.
The effect of non-ideal on-resistances and leakage currents can be seen in an exemplary circuit. FIG. 1 depicts an embodiment of a capacitance-to-voltage converter 20 that can include four switches 24, 28, 32, 36, an integrator including a differential amplifier A1 and a pair of capacitors CI1, CI2, and a sensing capacitor CS. In operation, two of the switches 24, 28 can charge the sensing capacitor CS to two different voltages. The capacitor charging time can be proportional to the on-resistance of these two switches 24, 28. The other two switches 32, 36 can connect the sensing capacitor CS to the inputs of the differential integrator. Leakage current in these other two switches 32, 36 may be integrated by the integrator, producing erroneous output voltages VOP, VON. The amplifier settling time may also depend on the on-resistance of these second two switches 32, 36, and the linearity of operation of the capacitance-to-voltage converter 20 may depend on any signal-dependent nature of these on-resistances.
The problems of a non-zero, signal-dependent on-resistance and a non-zero leakage current can be exacerbated by evolving fabrication processes used to manufacture transistors and switch circuits. Generally, a common trend in fabrication process evolution is for device sizes to scale downward in size, which can provide higher frequency operation and reduced power consumption. However, this scaling can also necessitate or result in reduced power supply and threshold voltages used with such transistors and circuits. Many performance measures of transistor-based circuits are dependent on the difference between the power supply and threshold voltages. For example, leakage currents of transistors can tend to increase when the threshold voltage decreases, but on-resistances of transistors can tend to decrease with decreasing threshold voltages. Thus, many transistor-based switch implementations face opposing performance impacts of device scaling.
A need exists, therefore, for a switch implementation that can exhibit both a low, signal-independent on-resistance when turned on and a low-leakage current when turned off, even in the face of fabrication process evolution in which device scaling may cause reduced power supply and threshold voltages.